Recently, due to increased direction to environmental preservation, electric vehicles (EV), hybrid electric vehicles (HEV) and fuel cell vehicles (FCV) are being developed. As a motor drive power source of these vehicles, a repeatedly rechargeable secondary battery is suitable. Particularly, non-aqueous electrolyte secondary batteries, such as lithium-ion secondary battery expecting a high capacity and high output are attracting public attention.
The non-aqueous electrolyte secondary battery has, as a component part, a power generating element that comprises laminated single-cell layers each including a positive electrode, a negative electrode and an electrolyte layer disposed between the positive and negative electrodes.
As the electrolyte layer, a structure including a separator made of a microporous resin sheet and a non-aqueous electrolyte such as liquid electrolyte or gel electrolyte held in the separator is known. In case of the lithium-ion secondary battery, the non-aqueous electrolyte contains organic solvent and lithium salt as essential components.
The separator that constitutes the electrolyte layer in each single-cell layer requires both a function to hold the non-aqueous electrolyte to secure an ion conductivity between the positive and negative electrodes and a function to serve as a partition wall between the positive and negative electrodes. Furthermore, in order to stop a charging/discharging reaction when the battery becomes a high temperature, it is desirable that the separator has a function to stop migration of ion, and hitherto, as such separator, a microporous film made of thermoplastic resin such as polyolefine or the like has been used.
However, in case of using a separator made of a soft material such as the polyolefine, there was a possibility that foreign pieces led into the battery during production of the battery and fragments of the electrode active material layer peeled from the electrodes would pass through the separator to induce undesirable internal short circuit.
In order to solve the above-mentioned problem, Patent Document-1 proposes an idea in which for suppressing such internal short circuit, a porous film made of a polyolefine or the like has at least one surface thereof a breathable surface protection layer (heat resistant insulating layer) including inorganic fine particles.
As a major step for manufacturing the non-aqueous electrolyte secondary battery having the above-mentioned structure, there is a process (lamination process) for producing a power generating element by laminating a plurality of single-cell layers each being produced by alternately laminating electrodes (positive electrode and negative electrode) and a separator. In this lamination process, the elements to be laminated should be precisely laminated or positioned in a lamination direction in order to prevent a lamination misalignment in a surface direction. If, in this lamination process, the elements are subjected to such a lamination misalignment, the active materials contained in the active material layers fail to be adequately used for the charging/discharging reaction, resulting in that a desired charging/discharging capacity is not obtained.
However, hitherto, means for sufficiently suppressing the above-mentioned lamination misalignment has not been known. According to the investigation by the inventor, it has been revealed that the lamination misalignment of the above-mentioned type takes place notably in an interface between the negative electrode active material layer and the separator.